1. Field of the Invention
The invention relates to a method and apparatus for detecting data signals in an optical disk system, and more particularly to a method and apparatus for detecting defect signals in an optical disk system.
2. Discription of the Prior Art
Because of the invention of the optical storage media, it is not difficult to store massive data. And because the optical storage media store data by recording data in the optical storage media in a digital way, data can be stored in a longer time compared with the conventional magnetic media in an analog way. In the meantime, the data will not be distorted with the time passing.
The earliest specification (red book) of the optical storage media was accomplished by Philip and Sony at 1980. Thereafter, many other specifications (e.g. yellow book, orange book . . . ) were finished for accommodating different contents. But basically, the storing formats of the optical storage media are the same, and they are expanded in accordance with the red book.
The data signal has to be modulated before stored in the optical storage media. For compact disk (CD), the data signal is processed in Eight to Fourteen Modulation (EFM) first. The modulated signal is a series of binary signals with the combination of logic “1” and logic “0”. If the binary signals are written into the CD directly, the pickup head of the optical storage media can not read the logic level states of every bit precisely, because the lengths of the binary pits are just about 0.13 um-0.15 um. Therefore, the modulated data signals have to be coded in the different lengths of pits (or lands) with lengths of 3T to 11T (3T, 4T, 5T, 6T, 7T, 8T, 9T, 10T, and 11T)(as shown in FIG. 1A), and stored in the optical storage media. The coding method of 3T to 11T is so called (2,10) RLL (Run Length Limited) coding (as shown in FIG. 1B). That means the data tracks of the optical disks are made of the helical tracks of pits with 9 different lengths. As shown in FIG. 2, 2A is a piece of data signal of the data track in an optical disk. RF signal 2B is reflected from the pickup head by tracking the data signals of the data track, while 2C is the NRZ signal corresponding to 2A in the (2,10) RLL coding. Because the reflected RF signal caused by the pits with different lengths has different signal amplitude intensities, the system can identify every data signal in accordance with its amplitude intensity. When the logic level of the NRZ signal in 2C changes (from the logic high level to logic low level, or from the logic low level to logic high level), it indicates logic “1”. When the logic level keeps the same, it indicates logic “0”.
Similarly, DVD (digital versatile disk) adopts the same method. The data signal is processed in 8 to 16 modulation (EFM plus), which are coded in 3T to 11T respectively and plus a 14T component, then the pits with different lengths are formed and stored in a DVD. For an optical disk, there is a plastic layer on its surface for protection, but it is likely to have defect signals produced by scratches or some exterior factors, such as the process of recording, fingerprints . . . etc. The defect signals cause the pickup head not to reflect the correct RF signals when tracking. Therefore, the optical disk system cannot read out the needed data signals and then cause false movements.
The RF signals produce irregular variation because of the existing defect signals. The conventional way of detecting defect signals is as shown in FIG. 3 which adds the reference voltage level 3B to the RF signals 3A reflected from data signals. When the voltage level of the envelope signal 3C of the RF signal 3A is lower than the reference voltage level 3B (as shown in the 3E area of FIG. 3), the defect flag signal 3D arises from the logic low level to logic high level to denote the 3E area has a defect signal, and inform the servo system of the optical disk system not to lock frequency to prevent from making a mistake.
But when the defect signals of the optical disk are not serious (e.g. defects caused by scratches and fingerprints), if the method described above is still adopted, it is possible the location of the defect signal cannot be found by the reflected RF signal from the data signal, as shown in FIG. 4. The voltage level of the defect portion in the envelope signal 4C of the RF signal 4A (as shown in 4E area of FIG. 4) is higher than the reference voltage level 4B in contrast. Thereby, the location of the defect signal cannot be found by the reflected RF signal from the data signal, and the defect flag signal 4D cannot reflect the appearance of the defect signal correctly.